The present invention relates to analog-to-digital converters. In particular, the present invention relates to resistor in analog-to-digital converters.
Analog-to-digital converters convert analog signals by sampling the signals and converting the samples into digital values. The sampling is performed by comparing the voltage of the analog signal against a set of reference voltages. Commonly, these reference voltages are produced using a resistor ladder consisting of a set of resistors of equal size connected in series between two reference potentials. The nodes between the resistors provide a set of voltages that are equally spaced between the upper reference potential and the lower reference potential.
In the prior art, the comparison between the input analog signal and the various voltages that are provided by the reference resistor ladder is accomplished using a plurality of comparators. Each comparator has one input coupled to a respective voltage node on the reference resistor ladder and a second input coupled to the analog input source that provides the analog signal. Since each comparator is connected to a different reference voltage, the output of the comparators will provide an indication of the voltage of the analog input signal.
Analog-to-digital converters of the prior art suffer from noise problems at the inputs to the comparators. Specifically, noise that is common to both inputs of a comparator can create a differential signal because of differences in the impedance attached to the inputs. The differences in the impedance seen by the inputs arise because one of the comparator inputs is coupled to the resistor ladder while the other input is coupled directly to the analog input source. If the differential signal created by the mismatch in resistor is larger than the smallest reference voltage on the reference resistor ladder, the analog-to-digital converter will generate the wrong digital value for the analog signal.